This application is the renewal of USPHS grant AM 18777. The proposed study is a continuation of research activities directed towards understanding the mechanism controlling the movement of potassium across colonic epithelium. Exeriments are designed to analyze the cellular events responsible for potassium transport in normal animals and in three models of increased potassium, glucocorticoid treated animals and states of hyperaldosteronism (e.g., Na depletion). Specifically, we hope to delineate the pathways for potassium movement, to determine the properties of the potassium pump at the basolateral membrane and to elucidate the cause of the increase in transepithelial potential differences. In addition, we plan to evaluate the relationship of sodium absorption to secretion of colonic potassium. Studies will also be performed to determine the mechanism of action of adrenal hormones, both glucocoticosteroids and mineralcocorticosteroids, on electrolyte transport and to correlate those changes in ion transport with occupancy of hormone specific binding sites. It is anticipated that these studies will provide a cellular model for colonic potassium secretion, an understanding of the forces responsible for an adaptive increase in secretion rate of potassium and insight into the mechanism by which adrenal hormones modulate ion movement across mammalian colon.